1. Field of the Invention
The present invention relates to sensors for critical stress diagnostics.
2. Description of the Related Art
Conductivity percolation, e.g., quasi-phase transition from a dielectric to conductive state, occurs in metal-dielectric composites in proximity of certain critical metal concentration. Electrical properties of near-percolation metal-dielectric composites are very sensitive to external pressure or internal stress, which makes them highly attractive for stress/strain sensors. Advantages of percolation-based sensors include a potentially broad range of detecting stress, strong change of conductivity under stress, and most importantly the possibility of direct detection of the dangerous tensile stress. A primary concern in applications for percolation-based sensors and in experimental research of metal-dielectric percolation, however, is producing a random metal distribution in the dielectric matrix. Additionally, Soft dielectrics, such as polymers, alkali-tungsten bronzes (like NaxWO3), or metal-ammonia solutions (like NaxNH3), cannot preserve their elastic properties over the important range of metal concentration. The last problem may be partly resolved in the mechanical mixtures of conducting and non conducting particles. However in mechanical mixtures as well as in common composites like Me—SiO2, Me—Al2O3 (where Me is Au, Ni or Al) it is difficult to reach an atomic-scale metal distribution. Heavy alloyed semiconductors, like Al—Ge, Pb—Ge, AlGe, or amorphous semiconductors combine both these problems as well as a principle question about applicability of percolation concept to the semiconductor's conductivity.
During a four-decade history of experimental research in metal-dielectric percolation starting from the initial works, a primary concern has been producing material with the random metal distribution in dielectric matrix. Many different composite structures were under examination, including metal-insulator mixtures and soft dielectrics, such as polymers, or alkali-tungsten bronzes (like NaxWO3) or metal-ammonia solutions (like NaxNH3). However, no one experimental system is uniform and stable enough to be compared with the percolation theory.
Still, the art suggests some applications of percolation phenomena for strain sensors. U.S. Pat. No. 6,276,214 (Kimura, et al.) discusses a strain sensor functioned with conductive particle polymer composites. When conductive particles are dispersed beyond the percolation threshold, electric conductive paths are formed between the electrodes by chains of particles contacting with each other between the electrodes. Elongation of this composite results in an increase in the gap distances between conductive particles. This results in the increase in the electric resistance of the composites. It is found that strain sensors can be made by the use of this nature. Strains of iron frames or iron-concrete are known by the change of electric resistance of the sensors which are set on a surface of the place to be monitored. The conductive particle-polymer composites are molded or printed and then endowed with electrodes so as to form strain sensors. The sensors are installed on surfaces of structural parts such as iron frames. Lead wires are connected to the electrodes of the installed sensors. It is necessary to know the places where the sensors are installed. Main fields of the application of the present sensors are safety monitoring systems for buildings, bridges, tunnels, dams, etc. The sensors are also applicable for tanks of chemicals, aircraft, ships and mega-floats.
U.S. Pat. No. 6,315,956 (Foulger) discusses Electrochemical sensors made from conductive polymer composite materials. An electrochemical sensor which is tailored for sensitivity to specific chemical analytes by selecting proper constituents. The electrochemical sensor is comprised of an immiscible polymer blend of at least two polymers in which a conductive filler is dispersed in one of the polymers of the blend through a multiple percolation approach to compounding. When in the presence of a chemical analyte, which is in either a liquid or vapor phase, one phase of the dual immiscible polymer blend swells, effecting a decrease in the conductivity, or increase in resistivity, of the polymer blend. The electrochemical sensor is reversible in that when the chemical analyte evaporates or is removed, the polymer blend returns to its original conductivity. With the multiple percolation approach it is possible to make a single composite material identifiably sensitive to various chemical analytes by incorporating several major phase materials into the immiscible polymer blend, each having an affinity for swelling for a different analyte. Further, the multiple percolation approach allows sensors to be made at extremely low cost.
The U.S. Pat. No. 6,452,564 (Schoen, et al.) discusses RF surface wave attenuating dielectric coatings composed of conducting, high aspect ratio biologically-derived particles in a polymer matrix. A coating composite is provided for a platform surface of an antenna array for, when applied to the platform, affording isolation of radiating and receiving antennas of the array. The coating composite includes a plurality of conductively coated elongate tubes dispersed in an insulating polymer matrix at a volume loading density approaching that at which the composite begins to conduct electrically over macroscopic distances, i.e., close to the percolation threshold. The tubes are preferably comprised of microtubules comprised of biologically-derived, high-aspect rod-shaped particles of microscopic dimensions having an electroless plated metal coating thereon.
However, besides the above described limitation of structural resolution and uniformity, the polymer-based conventional composites suffer from various thermal, mechanical and chemical impacts, and their applications for sensors, especially in aero-space industry are very limited.